gcc-6.3.0-2 has been uploaded for Cygwin. It is marked as a test
version.

JonY,
I assume you want some feedback on this.
I've been testing gcc-6.3.0-2 with binutils-2.28-3, on both x86 and
x86_64, by rebuilding several of my packages. So far I've built
texlive, emacs, and icu. There have been no problems, and all builds
have passed their test suites.
Ken